Book of Murder: Part 2 - S0.E10

Sebastian had "foreseen" his murder and the letter he had sent was addressed to reverend Jeremy Rathborne, calling him to investigate. Soon, it is found out that one of the murders was mistakenly done by a snake that was trained to go after Ciel's scent, later revealed to be sent by Snake from Noah's Arc Circus in an attempt to avenge his friends. The reverend also deduces that Siemens had jokingly faked his own death, but was killed soon afterwards by another guest – Woodley, who wanted to frame Ciel to prevent him from destroying his diamond business. The case is considered closed, but soon, Arthur Conan Doyle notices the inconsistencies in the theory and reveals the truth. Jeremy Rathborne is Sebastian. Sebastian admits and reveals the culprit's identity was known all along – it was Earl Grey, the Queen's butler, whom she had sent to kill Siemens, in order to disrupt Germany's financial stability, and at the same time, jokingly frame Ciel for the murder as a compensation for the accident with Noah's Arc Circus. Ciel and Sebastian simply made use of this occasion, knowing Siemens would be killed either way, and framed Woodley, who deserved to go to prison. For that, Sebastian managed to orchestrate the entire ordeal - do the Queen's secret task, clear the Phantomhive name of any possible murder accusations, and send a guilty businessman to jail. This experience influences Arthur Conan Doyle and he sets off to his career. At the end, Ciel invites Snake to live in the Phantomhive manor as well. Sebastian is made the Phantomhive main butler yet again and he swears his loyalty to Ciel until the day lies become truth.